Offset damping of electrical measuring instruments



May 19, 1953 E HABAKKE 2,639,307

OFFSET DAMPING OF ELECTRICAL MEASURING INSTRUMENTS Filed June 10, 1950 2 Sheets-Sheet 1 X h 90 O 80 FDEFLECT/O/V RIM/6E Inventor: Hans A. Bah he,-

His Attorney.

H. A. BAKKE May 19, 1953 OFFSET DAMPING OF ELECTRICAL MEASURING INSTRUMENTS Filed June 10, '1950 2 Sheets-Sheet 2 Inventor. Hans A.Bakke,

His Attorney.

Patented May 19, 1953 OFFSET DAMPING OF ELECTRICAL MEASURING INSTRUMENTS Hans A. Bakke, Swampscott, Mass, assignor to General Electric Company, a corporation of New York Application June 10, 1950, Serial No. 167,436

3 Claims.

My invention relates to electrical measuring instruments and, in particular, to offset dampin means therefor. The object of my invention is to modify and improve the magnetic damping characteristics of certain electrical measuring instruments in relation to the angular deflection in order that the damping will more nearly coincide with the angular position of the indicating pointer over the useful measurement range, in order to obtain a more nearlyuniform damping and response time over such measurement range. The invention is particularly useful in instruments which employ a nonuniform flux distribution as, for example, those instruments having a logarithmic response characteristic;

, The features of my invention which are believed to be novel and patentable will be pointed out in the claims appended hereto. For a better understanding of my invention, reference is made in the following description to the accompanying drawing in which Fig. 1 shows a plan view of pertinent portions of a logarithmic response type of measuring instrument to which my invention has been applied. Fig. 2 shows flux distribution, current, armature torque and damping torque curves plotted with respect to angular deflection, which'will be referred to in explaining the'invention as applied to an instrument such as represented in Fig. 1. Fig. 3 shows comparative response and overshoot curves plotted against ang'ular deflection before and after applying my invention to an instrument such as is represented in Fig. 1. Fig. 4 represents the application of my invention to a concentric scale type of instrumcnt. Figs. 5 and 6 represent difierent details of armature coil damping shell arrangements that may be used in carrying out my'invention. In'Fig. 1, I have represented portions of an electrical measuring instrument of a type used in logarithmic scale exposure meters. The stationary field is provided by an internal permanent magnet l of elongated shape with north and south poles at opposite extremities thereof. A stationary circular magnetic shell 2 surrounds the magnet l and is separated therefrom by an air 'gap and provides a flux return path for the permanent magnet l, the general flux path being indicated by dotted lines 3. The instrument is provided with a moving element pivoted on an axis 4 and includes the armature coil 5, pointer (i, ,dampingv shell I, and lead-in spirals which serve'alsoas zero return springs and one of which is shown at 8. The armature assembly with its pointer is shown in approximately the deenerstrument. The coil 5 is energized in such a direction that when energized, the armature will deflect in a clockwise upscale direction. The useful scale range is here assumed to be de-' grees. Assuming that the armature coil 5 is energized from a photoelectric cell for the purpose of measuring light values, the current will increase with increase in light intensity from zero to a maximum over the deflection range of the instrument, in general corresponding to the curve I, Fig. -2. In Fig. 2 the abscissas represent degrees deflection of the armature coil from maximum flux air gap position, and the ordinates represent maximum air gap flux density at different armature coil positions.

Now in order to produce a logarithmic scale deflection and a deflection torque curve corresponding approximately to the curve T, Fig. 2, it is necessary that the armature be cut by a maximum flux field near zero deflection where the armature current is very small, and by a minimum flux field near maximum deflection where the armature current is very high. Con+ sequently, a fleld flux distribution is provided in the armature air gap generally corresponding to the curve F, Fig. 2. This type of nonuniform air gap field distribution is provided by the magnetic field structure arrangement of Fig. l, where the coil 5 lies in the area of maximum or peak air gap flux when in the zero indicating position and as it turns clockwise from that position, progressively moves into areas of rapidity decreasing flux strength. Thus, the curves I, T, and that portion of curve F to the right of zero degrees may be taken to represent in general the current, torque and field flux of the general type of instrument here under consideration over the useful deflection range.

In instruments of this type and in permanent magnet field instruments generally as heretofore built,-instrument damping was often provided by using an armature coil supporting shell of conducting material, such as aluminum, which was centered with respect to the armature coil or coils so that the damping shell was always out by the same field flux as the armature coil or coils. Thus, if the instrument of Fig. l were provided with an armature supporting damping shell in alignment with the armature coil, we would obtain damping characteristics generally as represented by the dotted line curve (Z where there would be maximum damping at and near the zero armature deflection point, and negligible damping at and near the maximum deflection 99 some 50 e rees upscale from zero. Such damping is proportional to the square of the damping flux so that we would have more than ten times as much damping at five degrees deflection as we would have at 45 degrees deflection. Obviously, such damping characteristics are not satisfactory,

According to my invention, thedamp ng char acteristics of such an instrument are greatly improved by angularly offsetting the damping shell, relative to the armature coil in the direction of movement through the air gap and inaudirection. to increase the damping over the upperend: of the scale and decrease it ver the lower end of. the scale, and provide maximum damping over. the central deflection range where the instrument is likely to be most used.

Thus, in Fig. l, the damping-shell I is-displaced from the armature coil 5 about 25 degrees in a counterclockwise direction so that when the armature coil isatmidscale the damping shell willbe n he. max mum fl x fie f the a can and will; Obtain damping characteristics represented by theiullline curve B. It is noted that damping provided by the shell '5 has been decr ased atthel w n i he cal nd increa atthe pper. end of t es a t n urve D. as mpared. o. ur d andthat. in curveD thedamning at opposite ends, of the deflection range is about, one third maximum. This greatly improves, the dampin characteristics of the instrument. The curvesdandD are intendedto represent the main, instrument damping. which is provided by. armatureshells. The armature coil itself will generally provide some damping, but it is,relatively small andis not represented in Fig. 2, Thedamping shell'las ofiset from the armaturecoil as shown in Fig. 1 still provides support for, theaxialend, portions of the coil and pro-. vides,,means-for the axial support. of internal pivpts lead in. spirals, andthe like at and adjacentthe axis.,o f,rotation. The coil 5can readily. be madezsufficiently self-supporting by the insu lationused or asupplemental form or shell of insulatingmaterial to enable the invention to e used-Wi hou ifli ulty.

In Fig. 3,- 1 have represented comparison responsetimeand; overshoot curves for. instruments oi-thetyperepresented in Fig. 1 with and Without mprovement. n g, 3. th abscissas represent-degrees. deflect-ion where the, useful scale lengthis fio degrees The ordinatesv for response time curves-'1'; and R are on the; left in seconds response,- time from; zero. The ordinates for curyes o Qare-on the right in per centovershoot Glllve; risi-for, an instrument like Fig.1, xeen hav n s amn hell in l nment with its armature coil, and curve R is for an instrument with an; offset damping shell as in Fig. 1, From these curves it isnoted thatgif the in trumen e; ner iz d with rr nts. corresponding-to; deflections of. 40 degrees, it will take about; nine seconds for the pointer of the old instrument. to, come to rest, due toinsufficient. damping; for the deflection specified, whereas witnrny new. Clamping arrangement it will take the pointer only about 3.7 seconds to come to rest. The difference is less for lower deflections and' greater for larger deflections. It is further noted that my improved damping arrangement does not increasethe response time near zero deflection even though the damping in this region is very muchless than in the old arrangement. The per cent of overshoot for the old arrangement curve ois quite objectionable over the upper half,

of-'the scale where the instrument does not have enough damping and the overshoot varies from 30 to per cent, whereas with my improvement curve 0 the maximum per cent overshoot is 20 per cent at the extremities of the scale and is negligible over the center region of the scale.

Figs. 5 andfi illustrate moving. armature coils eqnippedwith my invention but: providing very satisfactory support for the coil with the same shell that supplies the damping. In Fig. 5, 1a represents the damping shell and 5a the armature coil, the two being effectively offset or displaced from each other in their direction of movement throughtheair gap- The damping torque curve for an. armature. of this type in an instrument otherwise like Fig.1 1 would be longer and flatter than the curves at and D of Fig. 2-. The armature of Fig. fiwhere lb represents the damping shell and; 5b the coil will produce damping generally corresponding to the curve D of Fig. 2, since most of the shell adjacent the air. can directly beneath the coil is cut away as represented at 9. When I mention the damping member being of!- set from the armature coil, I am referring to its effective damping action.

Fig. 4' represents my invention as applied to an instrument of the long concentric scale type. Here, It], represents the armature coil and H the damping shell which rotate inthe flux. air gap 32, aboutaxis 53. The air gap is bounded by the inner pole. piece It and outer pole piece I5. The air gap flux is furnished by a c-shaped' perma.- nent magnet I6 which is polarized radially. The inner pole piece It. is joined to the outer pole piece of the permanent. magnet by a magnetic ring H and split. magnetic tongue part l8. The air gap i2 is widened and theair gap flux weakened at the upper range of operation offthe armature to extend the range andreduce thesensitivity at the upper end of the scale. When the. armature coil Hi isdeflected into this weakv field range, as represented in the drawing, the damping shell. II is offset so that it remains in anair gaparea of high fiux density and thus provides sufficient damping even though the coil w. is in a.,weak field.

What I claim as. new and desire to secure by Letters Patent of the United. States is:

1. An electrical measuring instrument comprising a stationary field structure which CD11,- tains an air gap andheing provided with means for producing a fiuxacrosssaid gap, the flux crossing, said gap. having a, nonuniform distri bution, an armature coil mounted for movement through said air. gap from a region, of high. flux distribution to a region. of lower flux distribution over itsoperating range, and a clamping member having no, external circuit connections securedto said armature coil and also movable through said air gap, said armature coil and damping member being angularly offset in their; directions. of, movement by an amountv of the order. of /2 of the normal deflection range of said instrument sothat whenthe armature coil. is in a. region of low air. gap flux, concentration the damping member is in aregion of, higher air gap flux. concentration within the operating range.

2; An electrical measuring instrument. comprising a stationary field structure containing an air gap and means for producing, av flux through said gap, the flux crossing said gap being of nonuniform distribution, an armature coil'mountedgfor movement through said air gap from a region of high flux distribution to. a, on ffl w flux di ribution. as the oil moves through its operating range in an upscale direction, and a damping member having no external circuit connections secured to and moved with said coil in said air gap, said damping member being angularly offset from said coil in a downscale direction so that when the armature coil is in its upscale low air gap flux distribution range of operation the damping member is in a region of higher air gap flux distribution.

3. An electrical measuring instrument of the logarithmic scale distribution type comprising a stationary magnetic field structure which contains an air gap and means for producing a flux across said gap, the flux in said air gap having a maximum density at one portion thereof and decreases in density in opposite directions from said one portion, an armature coil mounted for movement in said gap, said coil being biased to occupy a substantially maximum flux density air gap position when deenergized and to deflect in an upscale direction from such position over its operating range through progressively decreasing fiux density air gap positions, and a damping conductor member, secured to and moved with said coil in said air gap, said dampposes.

HANS A. BAKKE.

References Cited in the file of this patent UNITED STATES PATENTS Number I Name Date 1,757,193 Hotopp May 6, 1930 1,933,327 Hoare Oct. 31, 1933 2,097,036 Mori Oct. 26, 1937 2,130,852 Lunas Sept. 20, 1938 2,566,783 Van Urk Sept. 4, 1951 FOREIGN PATENTS Number Country Date 321,826 Germany May 3, 1918 

